1. Field of the Invention
The present invention relates to an electromagnetic switch mounted on a starter for starting an engine.
2. Description of Related Art
Generally, an electromagnetic switch mounted on a starter is configured to generate a counter electromotive force across a coil depending on the inductance of the coil when a starter relay is turned off to interrupt a current flowing through the coil. If the counter electromotive force is applied to the starter relay, since an arc discharge occurs between the contacts of the starter relay, the contacts are worn, causing the life of the contacts to be reduced. Particularly, for the case of automobiles provided with an idling stop function, since the required life of the contacts is exceedingly long, it is critical to suppress occurrence of such an arc discharge to ensure a certain level of the life of the contacts.
As a measure to suppress such an arc discharge, it is known to connect a diode as a surge absorption element in parallel to the coil, as described, for example, in Japanese Patent Application Laid-open No. 2011-222410. The cathode terminal of the diode is connected to one end on the power source side of the coil. The anode terminal of the diode is connected to the other end on the ground side of the coil. Accordingly, when the starter relay is turned off to interrupt a current flowing through the coil, since a current generated due to the counter electromotive force across the coil is absorbed by the diode, the counter electromotive force can be prevented from being applied to the starter relay. Hence, it is possible to suppress wear of the contacts to ensure a necessary life of the contacts.
Meanwhile, it is known to clean the surfaces of the contacts of an electromagnetic relay by generating an appropriate arc discharge between the contacts to burn off oil or dust adhered to the contacts. However, in the case of the electromagnetic switch described in the above patent document where a diode as a surge-absorbing element is parallel-connected to the coil, since the current generated due to the counter electromotive force across the coil is absorbed by the diode, and accordingly the arc discharge between the coils of the starter relay becomes very small, there is a concern that the surface of the contacts cannot be cleaned sufficiently.